Within various sensors for measuring air quality there is a need to pass air through the sensor with a known volumetric flow. Such sensors include e.g. particle concentration sensors, humidity sensors and various gas sensors.
Fine particles are formed in many industrial processes and combustion processes. Furthermore, fine particles exist in breathing air flowing in ducts and ventilation systems and in room spaces. For various reasons these fine particles are measured. The fine particle measurements may be conducted because of their potential health effects and also for monitoring operation of industrial processes and combustion processes. The fine particles are also measured in ventilation systems for monitoring air quality. Another reason for monitoring fine particles is the increasing use and production of nanosized particles in industrial processes.
One prior art method and apparatus for measuring fine particles is described in document WO2009109688 A1. In this prior art method clean, essentially particle free, gas is supplied into the apparatus and directed as a main flow via an inlet chamber to an ejector provided inside the apparatus. The clean gas is further ionized before and during supplying it into the inlet chamber. The ionized clean gas may be preferably fed to the ejector at a sonic or close to sonic speed. The ionizing of the clean gas may be carried out for example using a corona charger. The inlet chamber is further provided with a sample inlet arranged in fluid communication with a channel or a space comprising aerosol having fine particles. The clean gas flow and the ejector together cause suction to the sample inlet such that a sample aerosol flow is formed from the duct or the space to the inlet chamber. The sample aerosol flow is thus provided as a side flow to the ejector. The ionized clean gas charges the particles. The charged particles may be further conducted back to the duct or space containing the aerosol. The fine particles of the aerosol sample are thus monitored by monitoring the electrical charge carried by the electrically charged particles. Free ions may further be removed by using an ion trap.
Operation of the particle sensor described in WO2009109688 A1 requires a clean air or gas source. Although in some special cases where the measurement interval is short a gas cylinder or equivalent can be used to provide the clean air, it is in most cases convenient to use some sort of a pump which is able to generate the required volumetric flow and operation pressure. When all the parameters stay essentially constant the construction described in WO2009109688 A1 provides an essentially constant sample flow through the sensor. However, if changes in operational parameters or other conditions may occur, such as changes in particle accumulation in the sensor, the volumetric flow through the sensor should be determined. WO2009109688 A1 is, however, silent on this.
One important demand for the fine particle monitoring apparatuses is reliable operation and efficient operation. Furthermore, it is also preferable that these fine particle monitoring apparatuses may be operated with low energy consumption and continuously for conducting fine particle measurements in real-time.
Whilst there exists conventional ways for generating the flow required for the operation of the sensors, such as fans, pumps or use of compressed gas, such solutions are sometimes not convenient due to e.g. frequently required maintenance. Thus there is a need for flow generation in a way which provides a long-term, reliable air flow.
U.S. Pat. No. 4,210,847, The United States of America as represented by the Secretary of the Navy, Jan. 7, 1980, provides a device for generating an air jet without the use of moving parts. High voltage is used to create a corona discharge electric wind in a ducted, compact, portable generator that can be used for augmentation cooling applications where high voltage is available.
The idea of using a corona discharge electric wind (also called “ion wind”, “ionic wind” or “corona wind”) in a sensor is provided in United States Patent Application Publication US 2011/0216317, Koninklijke Philips Elelctronics N.V., Aug. 9, 2011, describing a sensor which comprises a high-voltage discharge electrode for generating airborne unipolar ions that charge the airborne particles in the airflow. The generated ions are furthermore used to set-up an ionic wind between the discharge electrode and a counter electrode inside the sensor. The ionic wind is the driving force for maintaining the airflow through the sensor and allows sensor operation to occur free of audible noise. The presence of charged particles in the airflow is measured by an electrical current meter in the particle sensing section which measures the particle-bound charge that precipitates per unit time on the surface of a precipitation electrode after all airborne ions have been removed from air by a separate screening electrode positioned upstream of the particle sensing section.
Although the ion wind generated by a corona discharge unit may stay stable with clean air and with short time intervals, there is a tendency for the ion wind to change due to changes in corona geometry or corona tip soiling.
U.S. Pat. No. 3,324,291, Xerox Corporation, Jun. 6, 1967, describes the use of corona wind in a copying machine for generating an air flow which is utilized to provide a cleansing action by preventing the accumulation or deposition of dust in or about the unit. To ensure that the air entering the unit is itself generally free of dust or dirt particles likely to accumulate, there is provided a filter upstream of the corona wind generating unit. This will keep the air clean but while the filter gets loaded with dust, the pressure drop across the filter increases thus reducing the flow generated due to the ionic wind.
Even if the blocking filter problem might be solved by using an electrostatic precipitator, there still exists the feature that the air flow generated by an electric wind is usually quite low and due to small pressure differences e.g. flow through a sensor is sensitive to disturbances created by the sensor environment. Thus there exists a need for improved generation of known (i.e. identified) air flow using electric wind.